11 - Testing and characterisation techniques

Summary

Progress in our understanding of metal matrix composites is heavily reliant on the range of available experimental techniques and the correct selection by the researcher of the most appropriate method for a specific task. In this chapter an outline is given of some of the main testing and characterisation techniques which have been used for composite evaluation, looking first at the basic principles and capability of each technique, before going on to discuss application to MMCs. Each section also contains a source of references which provide the necessary technical details.

Measurement of Young's modulus

Basic principle and capability

The aim is to characterise the relationship between an applied load and a material's elastic (reversible) variation in strain. The available experimental approaches can be split into two broad classes; mechanical or static methods, and ultrasonic or dynamic methods. Their measurement capability in terms of accuracy is similar (±0.5 GPa).

Of all the different means of characterising stiffness, the following measures are perhaps the most popular.

1. (1) the tangent to the initial stress-strain slope

2. (2) the tangent, subsequent to prestraining, of the initial slope on reloading

3. (3) the tangent, subsequent to prestraining, of the initial slope of the unloading curve

4. (4) the tangent to the reloading curve, subsequent to prestraining and low stress amplitude cycling about zero until stress/strain hysteresis is negligible

5. (5) the speed of ultrasonic waves through the medium

6. (6) the frequency of resonant standing vibrations, which are related to the dimensions of the specimen as well as to the stiffness of the material.